Proposal: add `load` function to `google-chart-loader`

[rslawik]

Add the following function to the google-chart-loader module:

export function load(settings) {
  /* supply default values for settings and ... */
  /* delegate to Google Charts loader */
}

The options can specify:

• language - what language should be used by Google Charts library, default: lang attribute of the document or English if no attribute
• packages - which packages to load, default: ['corechart']
• version - what version of Google Charts library to load, default: 'current', other possible values
• mapsApiKey - used for queries from geocharts

The function returns a promise that resolves when the selected packages are loaded.

Use cases:

• preload certain packages
• show loading indicator while waiting for Google Charts library to be loaded
• support testing (e.g. await load() before constructing a test DataTable)

This is a part extracter from PR #204.

@wesalvaro, WDYT?